catch - Evaluate script and trap exceptional returns http://purl.org/tcl/home/man/tcl8.4/TclCmd/catch.htm catch script ?varName? '''`catch`''' ''`script`'' ?''`messageVarName`''? ?''`optionsVarName`''? The catch command may be used to prevent errors from aborting command interpretation. Catch calls the Tcl interpreter recursively to execute script, and always returns without raising an error, regardless of any errors that might occur while executing script. If script raises an error, catch will return a non-zero integer value corresponding to one of the exceptional return codes (see tcl.h for the definitions of code values). If the varName argument is given, then the variable it names is set to the error message from interpreting script. If script does not raise an error, catch will return 0 (TCL_OK) and set the variable to the value returned from script. Note that catch catches all exceptions, including those generated by [break] and [continue] '''and [return]''' as well as errors. The only errors that are not caught are syntax errors found when the script is compiled. This is because the catch command only catches errors during runtime. When the catch statement is compiled, the script is compiled as well and any syntax errors will generate a Tcl error. EXAMPLES The catch command may be used in an if to branch based on the success of a script. if { [catch {open $someFile w} fid] } { puts stderr "Could not open $someFile for writing\n$fid" exit 1 } The catch command will not catch compiled syntax errors. The first time proc foo is called, the body will be compiled and a Tcl error will be generated. proc foo {} { catch {expr {1 +- }} } (from: Tcl Help; bold words added by [RS]) ---- Why is it a bad idea to "catch" large chunks of code? Because Tcl stops execution of the code as soon as it encounters an error. This behavior can lead to problems like this: Why is it a bad idea to "catch" large chunks of code? Because Tcl stops execution of the code as soon as it encounters an error. This behavior can lead to problems like this: ====== # I've got an open socket whose handle's stored in fid catch { puts $fid "Here's my last message." close $fid } err If the "puts" command generates an error, "catch" detects that and stops execution of the code block. You never get around to closing the channel. It's better practice to put separate "catch" commands around If the "puts" command generates an error, "catch" detects that and stops execution of the code block. You never get around to closing the channel. It's better practice to put separate "catch" commands around both the "puts" and the "close" commands to detect errors in either case and handle them appropriately. This is a different style of programming than in a language like Java. In Java, you can have a variety of exceptions, each represented by a different class, that signal different types of error conditions. Then This is a different style of programming than in a language like Java. In Java, you can have a variety of exceptions, each represented by a different class, that signal different types of error conditions. Then in a "try" block, you can test for the different types of error conditions separately and handle them in different ways. (My complaint about Java is that there seems to be so many different classes, I'm often at a loss as to which ones I should be testing for.) In contrast, Tcl generally has only one type of error. In theory, we could use different return codes to signal different types of error, but in practice this is hardly ever used. ---- I think it is extreme parochialism to state "it's a bad idea to code this/that way". It is perfectly reasonable to catch "large" chunks: I think it is extreme parochialism to state "it's a bad idea to code this/that way". It is perfectly reasonable to catch "large" chunks: if { [ catch { close $fid } err ] } { } err ] } { return -code error $err } } If you catch "large" chunks you can at least have a program that can tolerate some errors that you did not anticipate. It is certainly better to fix problems the first time they appear, but tinkering with the code on a live system is rather poor practice. If you catch "large" chunks you can at least have a program that can tolerate some errors that you did not anticipate. It is certainly better to fix problems the first time they appear, but tinkering with the code on a live system is rather poor practice. Most users would rather ''not'' have the software come to a screeching halt on every unanticipated error. -'''PSE''' Most users would rather ''not'' have the software come to a screeching halt on every unanticipated error. -'''PSE''' ---- ====== set resource [some allocator] if {[set result [catch {some code with $resource} resulttext]]} { # remember global error state, as de-allocation may overwrite it set einfo $::errorInfo set ecode $::errorCode # free the resource, ignore nested errors # free the resource, ignore nested errors catch {deallocate $resource} # report the error with original details # report the error with original details return -code $result \ -errorcode $ecode \ -errorinfo $einfo \ $resulttext } deallocate $resource continue normally Besides a full-scale Java-style `[try]` implementation with all kinds of functionality for the catch branches, this could probably also be improved with a simpler command ''withresource'', that doesn't have the catch branch functionality, like this: Besides a full-scale Java-style [try] implementation with all kinds of functionality for the catch branches, this could probably also be improved with a simpler command ''withresource'', that doesn't have the catch branch functionality like this: ====== set resource [some allocator] withresource { some code with $resource } finally { # this would be executed always, even in case of error deallocate $resource } continue normally [HaO] 2012-02-17: Within the upper code, one may use the tcl 8.5 feature of an option dict returned by catch. ---- [MS]: ''Oops, forgot to mention it here: this bug was closed on 2000-10-25, it is fixed in tcl8.4a4.'' Note that the bytecode compiling of [[catch]] in Tcl 8.x has a bug. [[catch]] will inappropriately catch errors in the substitution of its arguments: % # should return 'foo' % catch [error foo] 1 Compare with Tcl 7.6: % catch [error foo] foo To work around the bug, defeat the bytecode compiler by generating [[catch]]: % [format catch] [error foo] foo Track Sourceforge bug # 119184 to learn if/when this bug is fixed. http://sourceforge.net/bugs/?func=detailbug&bug_id=119184&group_id=10894 '''DGP''' ---- Example for caught return, from a posting of [George Petasis] in [the comp.lang.tcl newsgroup]: ====== % proc foo {} { puts "catch result is :[catch { return}]" puts "after return" } % foo catch result is :2 after return ---- For a more complex worked example of catching [[break]], [[continue]], [[error]] and [[return]], this Wiki has a pure Tcl implementation of a Java-style [try ... finally ...] construct. -- [KBK] 24 Dec 2000 ---- When you [[catch]] the result of an [[exec]] command, ''$::errorCode'' contains a wealth of information about what happened. See the [exec] page for how to take it apart. ---- [MS]: this will work as long as the ''unknown'' proc has not been modified, and is relatively slow as the whole error processing machinery is also set in motion. If you want to use this approach in a more robust and fast manner, you may want to define ====== proc throw {{msg {}} {code 10}} { return -code $code $msg } This will throw an exception, caught by ''catch''; it will only be an error if code is set to "error" (or 1); do not use codes 0 to 4 in general as they have special meaning: 0 TCL_OK normal return 1 TCL_ERROR error return 2 TCL_RETURN cause the *caller* to return 3 TCL_BREAK call [break] in the caller 4 TCL_CONTINUE call [continue] in the caller ======none ---- See also [Tcl performance: catch vs. info] for the slowness of catch. ---- A common use for catch is when invoking external commands via [exec] . Since '''any''' non-zero return code would otherwise result in a raised error, one typically wraps the invocation with catch and then checks for the appropriate things. Since grep is just one example of a Unix command which uses a non-zero return code to mean something other than ''error'', one needs to be aware of cases like this when writing the exec. [[does someone have an idiom for execing a command, capturing the stdout, stderr, return code, etc. testing the return code and producing useful output based on stderr when appropriate, etc.? Answer: "[UNIX only exec wrapper]" is one model.]] ** `catch` and stderr ** ---- Someone mentioned on [comp.lang.tcl] that it took them a while to understand that when you use catch and supply a variable, the output from the command that would go to stderr ends up in the variable. Recently someone mentioned on comp.lang.tcl that it took them a while to understand that when you use catch and supply a variable, the output from the command that would go to stderr ends up in the variable. ---- See also [exec], [magic names], [errorCode] ---- [Tcl syntax help] - [Arts and crafts of Tcl-Tk programming] [Category Command]